The stellar life cycle describes the formation and death of stars. Stars begin as large clouds of dust and gas. These immense clouds (up to 25 light years in diameter) have a low density although each cloud contains as many particles as a single star. Hydrogen gas occupies as much as 99% of the cloud; less than one percent is made up of dust particles. The dust particles are tiny, each having a diameter of less than four hundred-thousandths of an inch (one ten-thousandth of a centimeter). These particles contain such compounds as silicon carbide and elements such as carbon and nitrogen. These clouds of dust and gas probably begin as a dying star exploding into a nova or a supernova.

The dust particles are gradually pulled close together by the force of gravity. An outside force, such as a shock wave from a supernova, usually...